1. Field of the Invention
The present invention is directed to a high-frequency power amplifier with a high-frequency input, a high-frequency output and at least one power transistor connected therebetween, of the type wherein the power transistor has a first electrode serving as control input, a cooling terminal connected to a second electrode and a third electrode.
2. Description of the Prior Art
Power transistors utilized in high-frequency power amplifiers require cooling for the elimination of dissipated power which can be considerable. Cooling terminals of the power transistors are usually fashioned as cooling vanes via which the transistors are thermally conductively connected to a cooling member. In many transistor types, the cooling vanes are electrically conductively connected to an electrode of the transistor, usually with the drain electrode in the case of field effect transistors and usually with the collector in the case of bipolar transistors.
Due to the required current and voltage gain, the power transistors are often inserted in the high-frequency power amplifier circuitry in a manner so that high-frequency voltages such as, for example, at the source or emitter circuit occur at the drain electrode or at the collector.
The cooling terminals, accordingly, carry a considerable high-frequency potential. A direct electrical connection of the cooling terminals to the cooling member intensifies the problems with the electromagnetic compatibility because the relatively large surface of the cooling member can emit high-frequency energy in this case. This must be additionally taken into consideration in the high-frequency shielding measures.
One possibility of reducing the outlay for shielding is to connect the cooling vanes of the transistors to the cooling member via a low-capacitance and thermally conductive insulation. The cooling member is thus electrically insulated from, first, the supply voltage and, second, the high-frequency. A disadvantage of this approach is that the waste heat must be eliminated via the electrical insulation layer. For reasons of cooling capacity, the electrical insulation layer should be optimally thin and have as large an area as possible. This, however, has the effect of increasing the output capacitance of the circuit, which can have a disturbing influence on the high-frequency behavior of the circuit.
When a source follower circuit (corresponds to the drain circuit), or an emitter follower circuit (corresponds to the collector circuit) is employed in the power amplifier, no high-frequency voltage is present at the drain or the collector electrode, respectively. Only the supply voltage is applied thereto. The high-frequency input voltage and the bias for the control electrode are present at the gate or base electrode. If the cooling member is to have no electrical potential, the electrical insulation of the drain or the collector from the cooling member only has to be designed for the supply voltage. Since these electrodes are free of high-frequency voltages, the insulation can be dimensioned to meet the cooling demands, i.e. the capacitance, and thus the area of the insulation, can in theory be arbitrarily large. A disadvantage is that this circuit has no voltage gain.